Enable --git-cache-dir option in chromite/cbuildbot.

scripts/slave/recipes/cros/cbuildbot_tryjob.expected/release.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--spec",
       "cache_dir = '[GIT_CACHE]'\nsolutions = [{'deps_file': '.DEPS.git', 'managed': True, 'name': 'chromite', 'url': 'https://chromium.googlesource.com/chromiumos/chromite.git'}]",
       "--patch_root",
       "chromite",
       "--revision_mapping_file",
       "{}",
       "--git-cache-dir",
       "[GIT_CACHE]",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
-      "chromite@master:",
+      "chromite@release-R55-9999.B:",
       "--output_manifest"
     ],
     "env": {
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "bot_update",
     "~followup_annotations": [
       "@@@STEP_TEXT@Some step text@@@",
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"did_run\": true, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"fixed_revisions\": {@@@",
-      "@@@STEP_LOG_LINE@json.output@    \"chromite\": \"master:\"@@@",
+      "@@@STEP_LOG_LINE@json.output@    \"chromite\": \"release-R55-9999.B:\"@@@",
       "@@@STEP_LOG_LINE@json.output@  }, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"manifest\": {}, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"patch_failure\": false, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"patch_root\": \"chromite\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"properties\": {}, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"root\": \"chromite\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"step_text\": \"Some step text\"@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
       "git",
       "checkout",
-      "master"
+      "release-R55-9999.B"
     ],
     "cwd": "[SLAVE_BUILD]/chromite",
-    "name": "checkout chromite branch [master]"
+    "name": "checkout chromite branch [release-R55-9999.B]"
   },
   {
     "cmd": [
       "git",
       "pull"
     ],
     "cwd": "[SLAVE_BUILD]/chromite",
-    "name": "sync chromite branch [master]"
+    "name": "sync chromite branch [release-R55-9999.B]"
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "[SLAVE_BUILD]/chromite/cbuildbot/config_dump.json",
       "/path/to/tmp/json"
     ],
     "name": "read chromite config",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@config@{@@@",
       "@@@STEP_LOG_LINE@config@  \"x86-generic-full\": {@@@",
       "@@@STEP_LOG_LINE@config@    \"_template\": \"full\", @@@",
       "@@@STEP_LOG_LINE@config@    \"build_type\": \"full\"@@@",
       "@@@STEP_LOG_LINE@config@  }@@@",
       "@@@STEP_LOG_LINE@config@}@@@",
       "@@@STEP_LOG_END@config@@@"
     ]
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "[SLAVE_BUILD]/chromite/bin/cbuildbot",
       "--buildroot",
       "[ROOT]/cbuild/external_master",
+      "--git-cache-dir",
+      "/b/cros_git_cache",
       "--timeout",
       "14400",
       "--remote-trybot",
       "--remote-version=4",
       "x86-generic-full"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "cbuildbot [x86-generic-full]"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]